Shorter bulb for operational speed and draught

Sep 02 2014

Sea waves have a significant impact on the vessels’ fuel consumption and are therefore one of the ship designers’ biggest challenges.*

When a vessel is sailing, waves are generated around the vessel due to its speed. This affects fuel consumption, as the vessel uses energy on generating the waves and because the waves increase the propulsion resistance of the vessel.

Not even the most skilled ship designers can prevent wave generation. But by altering the vessel’s design and further optimising it, it is possible to minimise the braking effect of wave generation against the vessel.

NORDEN’s two new Handysize product tankers, Nord Geranium and Nord Gardenia built by Guangzhou Shipyard International (GSI) in China, both have an optimised design with regard to counteracting the effect of wave generation.

Compared to the eight Handysize product tankers, which the southern Chinese yard delivered to NORDEN between 2006-2009, the latest two are both fitted with a 3-4 m shorter nose, or bulb. Not because there was something wrong with the design when the original vessels were constructed, but ship designers keep getting better at optimising vessel design.

The bulb plays a central role when it comes to counteracting the effect of the vessel’s wave generation as the bulb generates its own wave system around the vessel.

Wave systems offset each other “The observant reader will probably now think that if one wave system creates resistance, then two wave systems must create double as much resistance. But because the bulb’s wave system is generated suitably far in front of the hull, the bulb’s wave system with its crest and trough will be in opposition to the hull’s wave system.

“This means that the trough in the bulb’s wave system comes where the crest in the hull’s wave system is generated. Thereby, the two wave systems offset each other – more or less. At any rate, the bulb’s wave system reduces the braking effect of the hull’s wave system significantly. The extent of the reducing effect of the the bulb’s wave system depends on how well the design of the bulb fits the vessel’s actual speed and draught,” explained NORDEN’s senior newbuilding manager, Alex Hjortnas.

Right steaming

In recent years, vessels – drycargo, tanker and container vessels – have slowed down for commercial reasons. NORDEN calls this right steaming and it means that the bulb has to be shorter than before to be able to create a wave system, which is in opposition to that generated by the hull. With a bulb of the same length as earlier, the trough of the bulb’s wave system will come too far ahead to meet the crest of the vessel’s wave system when right steaming.

When GIS built NORDEN’s eight Handysize product tankers, it was very common that the bulb’s length and design in general was optimised in accordance with the service speed and design draught – ie the speed and draught which the yard’s designers considered most likely.

But it is one thing what the yard designers consider to be likely speed and draught once the vessels are in operation and another thing is the actual speed and draught of the vessels in operation.

“In realisation that many vessels only rarely sail with exactly the speed and exactly the draught which the yards’ designers have determined – typically the vessels sail at lower speed and less draught – the yards have started to optimise the bulb and the hull in general to a so-called operating profile.

“It is a combination of the speed and draught, etc representative of the market in which the vessel will be operating in and which in contrast to the old service speed and design draught, reflects practice and thus the real world,” said Hjortnas.

Nord Geranium is in operation and Nord Gardenia will follow shortly. “We are now looking forward to being able to measure the effect of the shortened bulb on the vessels’ fuel consumption,” Hjortnas concluded.

*This article was taken from NORDEN News.

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